Effects of ovulation rate and fetal number on fertility in twin-producing cattle.

Effects of ovulation rate and of fetal number and distribution within the uterus on pregnancy rate and fetal survival were evaluated in nulliparous (n = 1,331) and parous (n = 3,517) cattle selected for twinning. Cattle were divided into a spring (70 d) and fall (60 d) breeding season and bred by a combination of AI and natural service. Ovulation rate, pregnancy status, and fetal number and distribution were determined by transrectal, real-time ultrasonography of the uterus and both ovaries at the end of the breeding season. Pregnancy was reconfirmed by rectal palpation at 75 to 135 d of gestation. For heifers and cows combined, ovulation rate increased (P < 0.01) from 1.46 +/- 0.4 in 1994 to 1.89 +/- 0.4 in 2004; number of calves per parturition increased (P < 0.01) from 1.34 +/- 0.3 to 1.56 +/- 0.3, respectively, which included an increase in triplet and quadruplet ovulations and triplet births. Bilateral twin ovulations yielded proportionately more (P < 0.01) twin births than unilateral twin ovulations. Ovulation rate was greater (P < 0.01) in the fall than spring breeding season. Pregnancy rate at ultrasound diagnosis did not differ among females with 1, 2, or 3 ovulations (89.1 +/- 0.7, 91.2 +/- 0.7, or 91.5 +/- 2.8%, respectively), but rates at calving decreased (P < 0.01) with increasing ovulation rate (85.1 +/- 0.6, 82.7 +/- 0.6, or 64.2 +/- 2.7%, respectively). Pregnancy rate was less (P < 0.01) after twin or triplet births than single births. For dams birthing twins or triplets, pregnancy rate was less in the fall vs. spring, but rates were similar between seasons for dams with a single birth (type of birth x season, P < 0.05). Cows 60 d, regardless of type of birth. Maintenance of pregnancy to term differed (P < 0.01) among females diagnosed with 1, 2, or 3 fetuses (95.7 +/- 0.6, 87.8 +/- 0.8, and 54.9 +/- 2.3%, respectively). The reduced survival of twin and triplet fetuses in heifers had occurred (P < 0.01) by d 75 to 135 of gestation, and fetal losses were greater (P < 0.01) for unilateral than bilateral twins or triplets, whereas loss of twin or triplet fetuses in cows occurred later in gestation, and losses were not affected by uterine location. Thus, increased calf production from selecting for increased ovulation rate in beef cattle is tempered by increased fetal mortality, partially associated with the crowding of 2 or 3 fetuses within 1 uterine horn, especially in heifers.